Lithographic imaging systems are high precision, high cost optical systems. As the critical dimensions of the lithographic systems are decreasing, the imaging systems are subject to pressure to improve the accuracy of the images they form. Some lithographic imaging systems employ image correction to reduce errors in images they form. However, lithographic imaging systems with image correction are complex and difficult to align and to maintain in alignment. These complex systems are often subject to misalignment due to environmental thermal changes so that the imaging equipment must be maintained in a thermally stable environment.
FIG. 1 is a cross-sectional schematic side view of an Offner imaging system used in present day lithographic systems. The Offner imaging system 10 is a concentric imaging system having a primary mirror 12 and a secondary mirror 14. The primary mirror 12 has a concave spherical surface 13. The secondary mirror 14 has a convex spherical surface 15. The radius of curvature of the convex spherical surface 15 is about half the radius of curvature of the concave spherical surface 13. The convex spherical surface 15 and the concave spherical surface 13 have centers of curvatures positioned at about the same point 17 indicated by an X on the optical axis 16 shared by the primary mirror 12 and the secondary mirror 14.
In operation, an optical beam 21 propagating from object 18 located at the object plane 26 is directed towards primary mirror 12. Object 18 may be a spatial light modulator or other photolithographic reticle. The optical beam 21 is sequentially reflected by the concave spherical surface 13, the convex spherical surface 15 and the concave spherical surface 13. The second reflection by the concave spherical surface 13 directs the optical beam 21 out of the Offner imaging system 10.
The Offner imaging system 10 is a one-to-one imaging system and has an object plane 26 and an image plane 27. The Offner imaging system 10 forms a real inverted image 19 of an object 18 at an image plane 27 spatially removed from the object plane 26.
Reflection of the optical beam 21 by concave spherical surface 13 and convex spherical surface 15 produces no chromatic aberration. If the radius of curvature of the secondary mirror 14 is half that of the primary mirror 12, all 3rd order Seidel aberrations such as spherical, astigmatism, coma, field curvature and distortion are zero in the image plane 27. However, higher order astigmatism is problematic. Increasing the radius of curvature of the secondary mirror 15 from the 2:1 ratio with the radius of curvature of primary mirror 12 introduces some 3rd order astigmatism that cancels with the higher order astigmatism in a narrow annular region of the image plane 27. Any image formed within this annular region is well corrected. Unfortunately, the well-corrected region is a relatively small region of the image plane 27.
Rays that strike the center of the secondary mirror 14 are called chief or principal rays for the Offner imaging system 10. These principal rays propagate parallel to the optical axis 16 when entering or exiting the Offner imaging system 10. Each principal ray is the central ray of a bundle of rays propagating from the object plane 26 towards the image plane 27. Since the principal rays propagate parallel the optical axis 16, the region of the image side of the projection optical system 10 through which the principal rays propagate is within the well-corrected image region of Offner imaging system 10. Thus, the image 19 in the image plane 27 is well-corrected. Likewise, the region of the object side of the projection optical system 10 from which principal rays propagate is within the well-corrected object region of Offner imaging system 10 if the object 18 is located in the object plane 26.
The object plane 26 and the image plane 27 are equidistant from the point where optical beam 21 is incident on the convex spherical surface 15. If object 18 is moved towards or away from the concave spherical surface 13 on the optical path of the principal ray entering the Offner imaging system 10, the image 19 formed in an image plane 27 is moved an equal distance towards or away from, respectively, the concave spherical surface 13 on the optical path of the principal ray exiting the Offner imaging system 10.
A spatial light modulator located in the object plane 26 is imaged in a one-to-one dimensional relationship on a workpiece. The workpiece may be, for example, a wafer located at the image plane 27.
Some digital photolithography systems use a reticle that is dynamic, not fixed. In such systems, light is reflected at, transmitted through or emitted from a spatial light modulator located in the object plane 26. The spatial light modulator has a high aspect ratio. The aspect ratio of the spatial light modulator is the ratio of the length to the width of the spatial light modulator. A high aspect ratio is a ratio of more than 5:1. A typical spatial light modulator for a digital lithography system has dimensions of 75 mm to 1 mm for a 75:1 aspect ratio. To fit the complete image of the spatial light modulator within the well-corrected annular region of the image plane 27 requires that the diameter of the primary mirror 12 be large. For a 1 mm by 75 mm reticle image to be within the well-corrected annular region of the primary mirror 12, the diameter of the primary mirror would be about 651 mm. A concave mirror of this size is very expensive.
A projection optical system for digital lithography is described in U.S. patent application Ser. No. 10/933,170 of Russell W. Gruhlke, et al. entitled Offner Imaging System with Reduced-Diameter Reflectors filed on Sep. 2, 2004. In patent application Ser. No. 10/933,170, the projection optical system includes an Offner imaging system defining an optical axis and having a well-corrected region and means for shaping an optical beam having an extent too large to fit within the well-corrected region to propagate through the Offner imaging system within the well-corrected region.
What is needed is a way to reduce the diameter of the primary mirror 12 in an optical imaging system for digital lithography capable of imaging low or medium aspect ratio reticles within the well-corrected region.